The present invention relates to programmable circuits for electronic musical instruments. More particularly, the invention relates to programmable dynamic filters for use with time-shared electronic organs having frequency response characteristics which are controllable independently of the audio frequency of the musical note being played.
Conventionally, electronic musical instruments such as organs and synthesizers employ tone generators of two general types. Synthesizers have generally utilized a voltage-controlled oscillator, the control voltage applied thereto being a function of the note to be sounded. These systems are monophonic, being adapted to sound only a single note at a time. Organs have normally utilized either a separate oscillator for each note which is to be generated or a single oscillator whose output is passed through suitable dividers to obtain signals at each frequency which the instrument will be required to sound. Separate keyer circuits, envelope generators and filters are employed to suitably shape the tone signals, and many such circuits are required to produce a variety of different tones and voices. These circuits are each dedicated to respective audio tone signals of particular frequencies in order that their timing characteristics may be "tuned" to those frequencies. As a result, hundreds of circuits are required in a prior art organ. This multiplicity of circuitry significantly affects the size, weight, complexity, power consumption and cost of the instrument.
Recently, time-shared systems have been proposed which greatly reduce the number of circuits which are required to implement an electronic organ. One such system is disclosed in co-pending application Ser. No. 835,832, filed Sept. 22, 1977 in the name of Richard S. Swain et al, entitled "Tone Generating System for Electronic Musical Instrument" and assigned to the assignee of the present invention. A variety of programmable circuits for use with the Swain et al system are disclosed in co-pending application Ser. No. 835,695, filed Sept. 22, 1977 in the name of Glenn Gross, entitled "Programmable Circuits for Electronic Musical Instrument" and also assigned to the assignee of the present invention.
In the time-shared electronic organ disclosed in the foregoing co-pending applications, a limited number, preferably ten or twelve, of musical note-sounding channels are provided. Each channel, which typically consists of a priority note generator, an envelope generator and driver, a programmable keyer and a programmable filter, can be assigned as needed to sound any note in the entire musical range of the instrument by the use of time-shared techniques. Moreover, to further reduce circuit multiplicity, the frequency dependent circuits of each channel, such as the filters, are programmable to facilitate the use of a single filter for processing all the musical tones which may be produced by a particular note generator. In other words, the frequency response of a channel filter is preferably tailored according to the frequency of the note being generated in order to maintain proper musical characteristics. Thus, for example, the filter would exhibit a relatively high frequency range for the higher pitch musical notes produced by the tone generator and a lower frequency range for the lower pitch notes. This is accomplished by generating an encoded signal identifying the octave or half-octave in which a generated tone signal is contained and programming the filter in accordance therewith. The filter is thereby operated to exhibit an appropriately different response depending upon the frequency of the generated tone signal, this technique being commonly referred to as "tracking." Each different frequency response is characterized by a respective frequency range normally referred to as the tracking interval of the filter.
However, in order to achieve certain musical effects, it may be desirable to program the channel filter for modulating the filter's frequency response or tracking interval beyond or in addition to the response achieved from slavishly tracking or following the tone being played. Stated otherwise, the channel filter should be programmable for initially tracking the tone being played and should further be independently controllable whereby this initially tracking interval may be adjusted or modulated to produce various desired effects.
Various techniques for modulating a filter beyond its initial tracking interval corresponding to a tone being sounded are known in the art. In one such system, an exponential converter operates a voltage-controlled filter, the filter having an input for receiving a generated tone signal. The exponential converter has two inputs, one input being connected for receiving a DC signal corresponding to the selected note and the other receiving an independently derived modulating signal. The output of the converter is therefore an analog signal operating the voltage-controlled filter in response to both the note being played and the independently derived modulating signal. Systems of this type, being analog in nature, are not readily suited for inclusion in a time-shared digital electronic organ of the variety disclosed in the previously mentioned co-pending applications.
U.S. Pat. No. 3,974,461 to Luce discloses another prior art filter which may be operated for modulating its tracking interval beyond an initial tracking interval corresponding to a note being played. In the Luce system, a filter is provided consisting of a series of gates interposed between a plurality of individual low-pass filter sections. The frequency response or tracking interval of the aggregate filter is therefore determined by the conduction states of the gates. Control of the gates is affected by the output of a monostable multivibrator which develops output pulses of fixed duration at a repetition rate determined by a voltage-controlled oscillator connected to its input. The voltage-controlled oscillator is, in turn, operated in response to an analog input signal derived from a summing circuit which may combine a keyboard related analog signal as well as an independently generated modulating signal. It will be appreciated that the repetition rate of the pulses produced by the monostable multivibrator, which is ultimately dependent upon the analog signal developed by the summing circuit, controls the frequency response and therefore the tracking interval of the filter. This signal, which is partly digital in nature and partly analog, is also not readily suited for incorporation in a completely digital time-shared instrument. Furthermore, quite contrary to the teachings of this reference, the elimination of voltage-controlled ocillators is one of the primary purposes of time-shared systems.